Arrange the following in decreasing order of their boiling points:
[A] n-butane
[B] 2-methyl butane
[C] n-pentane
[D] 2, 2-Dimethyl propane

Question

Arrange the following in decreasing order of their boiling points: 
[A] n-butane 
[B] 2-methyl butane 
[C] n-pentane 
[D] 2, 2-Dimethyl propane

Accepted Answer

To determine the correct order of boiling points for the given compounds, we need to consider the molecular structure and the factors that influence boiling points, such as molecular weight and branching.

1. **n-Pentane (C):** n-Pentane is a straight-chain alkane with five carbon atoms. Straight-chain alkanes generally have higher boiling points than their branched counterparts due to greater surface area, which leads to stronger van der Waals forces (London dispersion forces). Therefore, n-pentane is expected to have the highest boiling point among the given compounds.

2. **2-Methylbutane (B):** 2-Methylbutane is a branched alkane with five carbon atoms. The branching reduces the surface area compared to n-pentane, leading to weaker van der Waals forces and a lower boiling point than n-pentane.

3. **2,2-Dimethylpropane (D):** 2,2-Dimethylpropane is a highly branched alkane with five carbon atoms. It has even more branching than 2-methylbutane, resulting in a more compact shape and a further reduced surface area. This leads to even weaker van der Waals forces and a lower boiling point than both n-pentane and 2-methylbutane.

4. **n-Butane (A):** n-Butane is a straight-chain alkane with four carbon atoms. Although it is a straight-chain compound, it has fewer carbon atoms than the other compounds, resulting in a lower molecular weight and weaker van der Waals forces compared to the five-carbon alkanes. Therefore, n-butane has the lowest boiling point among the given compounds.

Based on this analysis, the correct order of boiling points from highest to lowest is: n-pentane (C) > 2-methylbutane (B) > 2,2-dimethylpropane (D) > n-butane (A). This corresponds to Option 4: $C>B>D>A$.

**Explanation of Incorrect Options:**

- **Option 1 ($A>B>C>D$):** This option suggests that n-butane has the highest boiling point, which is incorrect because n-butane has the lowest molecular weight and fewer carbon atoms, leading to a lower boiling point than the other compounds.

- **Option 2 ($B>C>D>A$):** This option incorrectly places 2-methylbutane above n-pentane. As explained, n-pentane, being a straight-chain alkane, has a higher boiling point than the branched 2-methylbutane.

- **Option 3 ($D>C>B>A$):** This option suggests that 2,2-dimethylpropane has the highest boiling point, which is incorrect due to its highly branched structure, resulting in the lowest boiling point among the five-carbon alkanes.

Thus, Option 4 is the correct answer, as it accurately reflects the decreasing order of boiling points based on molecular structure and branching.